As shown in FIG. 11, an optical disc 51, which became an indispensable information medium recently, has a ring-shaped clamping area 53, which stores no information, formed outside a central aperture 52, a ring-shaped information recording area 54 formed outside the clamping area and a transparent area 63 with a small width, which stores no information, formed at an outer disc periphery at the outside of the information recording area. Such standard-sized optical disc is formed into a true-circular shape with a diameter of 12 cm (weight: 15 to 16 g) or a diameter of 8 cm (weight: 7 to 8 g).
Other than the standard-sized optical disc 51, a card-shaped optical disc 70 or 80 having the same size as that of a card such as a credit card has been proposed recently by, for example, Japanese registered utility model No. 3,064,414.
One of the proposed optical discs 70 is shown in FIGS. 12 and 13. The entire disc including a disc main body 71 is formed into a card-shape having the same size as that of various cards such as a credit card. The thickness “t” is set to 1.2 mm that is a standard thickness of an optical disc. The disc main body 71 is provided with a pair of arc-shaped protrusions 75 and 75 at the lower side and outside the information recording area 74 of the disc main body. The arc-shaped protrusions 75 and 75 can fit in the 8 cm-diameter disc main body fitting dented portion 92 formed in the tray 91 of the disc player 90 as shown in FIG. 16 and the radius of curvature is set to about 4 cm.
Furthermore, an optical disc 80 according to another proposal is shown in FIGS. 14 and 15. In this disc, the thickness of the disc main body 81 including the information recording area 84 is increased while keeping the thickness of the external plate 89 formed outside the disc main body 81 the standard thickness, and the external periphery of the disc main body 81 is formed into an arc-shape having a radius of curvature of 4 cm.
Both of the aforementioned optical discs 70 and 80 have such advantages that it is possible to carry them in a wallet because they are card-shaped discs having the same size as those of various cards such as credit cards and that they can use as business cards depending on the contents of information recorded therein or printed information formed on the label side.
Although these optical discs 70 and 80 have aforementioned advantages, they also have the following disadvantages. That is, the disc becomes bulky due to the thickness of the arc-shaped protrusion 75 and the increased thickness of the disc main body 81, and therefore if a plurality of these optical discs are pilled up and stored in a wallet or the like, the total thickness increases although each increased thickness is small.
Furthermore, if the increased thickness is reduced in order to lessen the aforementioned disadvantage as much as possible, the stability of the optical disc in the disc main body fitting dented portion 92 deteriorates, which may cause inaccurate positioning of the disc in the disc player 90.
In order to improve the stability of the optical disc in the disc main body fitting dented portion 92, it is preferable to further increase the increased thickness. However, in a slot-in type disc player, if the entire thickness of the optical disc exceeds 1.5 mm, even if an adapter designed for card-shaped discs is employed, loading becomes impossible. Accordingly, in order to cope with the slot-in type player, the upper limit of the increased thickness is 0.3 mm.
However, in cases where the increased thickness is only 0.3 mm, the optical disc may not be installed in a tray type disc player 90. In other words, in cases where the disc player 90 is disposed horizontally, since the disc placing side of the tray 91 becomes horizontal, there is no problem. To the contrary, in cases where the disc player 90 is disposed vertically, since the disc placing side of the tray 91 will be also vertical, only the 0.3 mm increased thickness portion of the optical disc can be engaged with the disc main body fitting dented portion 92 of the vertically disposed tray. Accordingly, the optical disc 70 may drop out of the tray 91 at the time of the retraction movement of the tray into the disc player 90.
Furthermore, even in cases where the disc player 90 is disposed horizontally, if the moving speed of the tray 91 becomes faster, the optical disc may drop out of the disc main body fitting dented portion 92.
Furthermore, since the arc-shaped protrusion 75 or the thickness increased portion has to form on the disc main body 71 or 81, it is impossible to manufacture such optical discs 70 and 80 by cutting a 12 cm-standard-sized optical disc substrate into a desired shaped and grinding it.
The present invention was made to solve the aforementioned objects, and aims to provide a nonstandard-sized optical disc capable of reducing the thickness as much as possible even when a plurality of the optical discs are piled up and easily being fit in a disc main body fitting dented portion of a disc player.